THE BACTERIA 201 
188. How bacteria secure their food. Since they are so sim- 
ple in structure, and since they live in direct contact with 
their food supply, the bacteria absorb their nourishment directly 
through their cell walls. Different kinds of bacteria may live 
upon different kinds of organic matter, but almost every kind 
of organic matter may serve to nourish some kind of bacteria. 
Certain kinds of bacteria can thrive only in absence of free oxy- 
gen, and a few others can construct food somewhat as green 
plants do. It must also be noted that bacteria, like other liv- 
ing things, produce and excrete substances which, if retained, 
would be injurious to them. If excreted and accumulated 
about the bacteria in great quantity, these substances would 
soon kill them. If a jar of beef broth is carefully sealed after 
any ordinary bacteria have been introduced into it, there will 
at first be a rapid increase in their number, and the liquid will 
become clouded with the organisms and their products. But 
the excretions soon accumulate to such an extent that the 
bacteria can no longer grow. They become dormant or may 
die and settle to the bottom of the jar or collect in a jelly-like 
mass at the surface. 
189. How bacteria reproduce themselves. When in favor- 
able nutrient material, bacterial cells divide frequently. A 
plant thus forms two new ones, each of which may soon (in 
from twenty minutes to half an hour) divide again. This is the 
simplest possible method of reproduction, consisting merely 
of the dividing, or fission, of a single-celled plant. The possi- 
bilities of this rate of reproduction are enormous. If all con- 
ditions were to remain entirely favorable for reproduction, a 
bacterium which divides but once an hour would in two days 
produce offspring numbering 281,500,000,000, and “in three 
days the progeny of a single cell would balance 148.356 
hundred-weight.” ! Of course it is well known that ordina- 
rily this rate of reproduction cannot be realized, because 
growth conditions do not remain favorable. The food supply 
1 Jordan, E. O., General Bacteriology, p. 62. W. B. Saunders Company, 
1911. 
